If the part is not symmetrical and you have a mirror of it, it would not be the same part, so SW will not list it as the same item. If you are just using the mirror to locate a new part but it actually is the exact same part, then you deselect the "Create opposite hand version" box in the mirror command.

Or you could just insert the part again and locate it with mates.

If you actually do need an opposite hand mirror but want it as one item in the BOM then you have a couple options. If you created the mirror as a configuration within the same part, then in your BOM options under Part Configuration Grouping you can select the bubble for "Display all configurations of the same part as one item".

Or if you created the mirror as a separate part file, then you can make the configuration name of the normal part and the mirrored part the same name, then check the next bubble for "Display configurations with the same name as one item". However if you have other parts with the same configuration name (like several parts with only a "Default" configuration) then those will merge into one BOM item as well.